1. Field of the Invention
This invention relates to a method of operating an extrusion line having a single roller die, and more specifically to a method in which the size and shape of the extrudate are controlled by varying extruder operating speeds.
2. Description of the Prior Art
An extrusion line with a roller die comprises an extruder which heats, breaks down, mixes, and transports material to a reservoir. A roller die includes a roller which conveys the material from the reservoir by rotational motion past a plate die which shapes the material to produce an extrudate. An example of such an extruder and roller die combination may be found in U.S. Pat. No. 3,871,810, issued to Geyer. The extrudate is then transported from the roller by a take-away device or other conveyor system, while allowing it to stretch or contract and ultimately solidify by cooling or cross-linking.
In a cold-feed extrusion line, the output rate of the extruder is determined primarily by the speed of the screw which rotates inside the extruder. The screw speed governs the rate at which the material in the extruder can be advanced to the roller die. In a hot-feed extrusion line, the output rate of the extruder is determined primarily by the feed rate of hot stock which is supplied from a feed mill located some distance from the extruder.
While the extrusion line is designed to produce an extrudate having specified dimensions, deviations from the specified dimensions are continually being introduced by changes in material properties or by variations in the extrusion line operation. The dimensional variability can be divided into two categories: first, size deviations in which the cross-sectional area changes, but in which the ratios of the width and thickness dimensions (i.e., the shape) are constant; and, second, shape deviations in which the ratios of the width and thickness dimensions change, but in which the cross-sectional area (i.e., the size) is constant. In size changes, all dimensions change by the same percentage. In shape change, the dimensions change by different percentages from specification, although the overall cross-sectional area remains the same.
The control of size deviation by manipulating the output rate of the extruder or by manipulating the speed of the take-away device has been well known in the art. In cold-feed extruders, it is known to manipulate the speed of the screw to vary the extruder output and thus control size deviations. Likewise, in hot-feed extruders, it is known to manipulate the feed rate by varying the width of the strips of rubber fed into the extruder from the breakdown mill. Examples of processes in which the take-away speed is manipulated to control size deviations are also found in U.S. Pat. No. 4,087,499, issued to Bayonnet; No. 4,088,721 to Apicella; No. 4,097,566 to Bertin et al.; and No. 4,233,255 to Moon. Another example of a process control in which a double roller die is used to control size is shown in U.S. Pat. No. 3,975,126, issued to Wireman et al.
Shape deviations, however, have been more difficult to control by known means. One way of altering the shape has been to alter the geometry of the die by removing or adding metal along the die opening. This method is difficult to use during a production run because it requires production to be shut down during die modification and testing, and it causes a physical deterioration of the die.
Another approach for controlling shape deviations has been to use a die which could be adjusted by heated bolts or movable die sections. This method has often been used for flat or annular dies. For typical production profile dies, however, this method required a very complicated mechanical device. For tire tread extrusion, the adjustable sections of the die would also cause discontinuities along the tread profile.
Another more recent approach to controlling shape deviations is disclosed in U.S. patent application Ser. No. 06/412,634, filed simultaneously with this application, by Lee, Stevenson, and Griffith. This method proposes the manipulation of extruder operating temperatures in order to control shape deviations, with either the extruder stock temperature or the extruder die temperature, or both, being varied.